The invention relates to an improvement in a sewing machine having a table, driven in one plane and carrying the material to be sewn, a stitch-forming device and a cutting device for producing a buttonhole in the material to be sewn. The buttonhole is provided by an incision, and by zigzag stitches forming a buttonhole bead running around the incision. The incision is produced either before the buttonhole bead in a pre-cutting mode or after the buttonhole bead in a post-cutting mode. The sewing tools comprise a needle bar, which is driven up and down and oscillates in the horizontal direction, and a needle, which is provided at the bottom end of the needle bar and interacts with a rotationally driven looper mounted in the base plate.
An eyelet-buttonhole machine has been sold for many years by the applicant under the designation "Durkopp Adler Kl. 558". In the pre-cutting mode, the buttonhole is cut first and then the buttonhole bead is produced around the incision. In the pre-cutting mode, the aim is to ensure that opposite seam rows lie exactly next to one another, so that fraying of the cut-open material is prevented. In the post-cutting mode, first the buttonhole bead is produced and then the buttonhole is cut. In the post-cutting mode, an intermediate space (intermediate material) must remain between the two opposite stitch rows forming the buttonhole bead, so that during the subsequent cutting of the buttonhole only the material to be sewn is cut and not the sewn buttonhole bead.
To produce the two different bead positions required in the pre-cutting and post-cutting modes, respectively, both the motion of the needle bar and the motion of the table are controlled in this machine. Both the stitch position affecting the intermediate material and the stitch width can be set on this machine. The table is arranged so as to be longitudinally movable and rotatable or pivotable below the needle. The longitudinal motion is controlled by a main cam plate and the pivoting motion is controlled by an eyelet control disk. The pivoting motion is necessary so that the material can be laterally displaced below the needle in order to thereby produce the eyelet of the buttonhole bead. By selection of a particular eyelet control disk, the pivoting motion of the table is controlled during the forming of the eyelet, so that the width of the eyelet can be varied independently of the forming of the stitches.
The operability and the functional reliability of a machine controlled in such a way are very high. The exchange of the control disks for various buttonhole shapes, however, is time-consuming and therefore costly. In addition, this sewing machine is of very complicated mechanical construction, which results in a large number of different individual parts. The large number of parts requires not only a corresponding cost-intensive assembly effort but also elaborate stockkeeping, as a result of which the manufacturing and spare-parts costs increase.
DE 41 32 586 C2 discloses an eyelet-buttonhole sewing machine in which the intermediate material is provided by displacing the sewing pattern for producing an offset. This offset is imparted to the transport table. That is to say, before the forming of the stitches is started, the transport table is put into such a position that the inner stitch of the needle, provided opposite the subsequent incision point, maintains a corresponding distance from the opposite stitch. To this end, the table is driven by two stepping motors, which are arranged along respective axes (X, Y) disposed perpendicularly to one another. By the storage of different data records which are used to control the stepping motors, the offset can be activated according to operating mode.
A disadvantage with this machine is that two sets of activating data for the X- and Y-motors have to be re-calculated as a function of the desired offset value and the desired dimension of the intermediate material. If steps are lost when approaching the initial position, there is the risk of the stitches being staggered in the pre-cutting mode and of the already-formed buttonhole bead subsequently being cut open as a result.
German Patent 690 654 discloses a buttonhole sewing machine having a main cam plate for controlling the longitudinal motion of the material-supporting plate, on which the material to be sewn lies. The motion of the table for forming the buttonhole eyelet is controlled by an interchangeable auxiliary cam plate mounted outside the housing on a rotatable supporting pin passing through the machine housing. The exchange of the auxiliary cam plate for forming different buttonhole eyelets is quite complicated.
U.S. Pat. No. 1,991,627 discloses a buttonhole sewing machine which is provided with a device for adjusting or changing the position of those stitches of the zigzag stitches which face one another in order to produce the intermediate material within the buttonhole bead which is necessary in the post-cutting mode.
Reference may also be made to German Patent 16 60 845, which discloses a twin-thread chain-stitch buttonhole sewing machine in which the overstitch width is automatically adjustable before the sewing of the crossbar. To this end, a device which uncouples the drive mechanism producing the zigzag motion of the needle from the manual setting and sets it to a predetermined value of the overstitch width is actuated when a certain rotary-angle position of the stitch-pattern tools is reached. A change in the stitch width position by program over the entire stitch width for an eyelet buttonhole is not possible.
The disclosures of all prior art mentioned herein are expressly incorporated by reference.